1. Field of the Invention
The invention relates to multi-disk storage systems in which continuously rotating closely packed flexible disks are randomly accessed.
2. Description of the Prior Art
Rotating magnetic disk memory systems and in particular rigid disk types have been used to provide large amounts of data storage capacity for computer processing. However, the operation of the rigid disk system requires a plurality of read-write transducers to be simultaneously moved to a particular radial read-write track position between the disks which has reduced the volumetric efficiency of this system.
In order to overcome the rigid disk inefficiencies by increasing the number of bits (i.e., binary information) per unit volume, compliant or flexible disks have been utilized. A random access flexible disk memory system is characterized by a closer spacing between adjacent disks but since they are compliant there is a tendency of the disk to "flop" (i.e., to move outside of its normal plane during low speed rotation). In view of the above described characteristics of the flexible or compliant disk, rapid accessing of a random disk in a stack with minimum error, that is, without having to make several selection attempts before success is obtained, is not readily obtainable.
The prior art discloses several attempts to enhance the accessing of the closely packed flexible disk memory. In patent U.S. Pat. No. 3,931,645 there is disclosed a flexible disk file wherein disks are arranged into a plurality of groups wherein each group is bounded by two disks having a maximum radius with at least two of the disks in each group having a minimum radius and at least one additional disk having an intermediate radius interposed between such minimum radius disks. Accessing of a particular disk is achieved with a partitioning arm which follows a certain weave motion. The principle of this invention is that a window is provided between two maximum diameter disks for accessing either the largest, intermediate or smallest disks by use of the weave motion.
An accessing technique for a flexible disk memory is also disclosed in an I.B.M. Technical Disclosure Bulletin, Vol. 12, No. 1, June 1969. The disk system utilizes different diameter disks which vary uniformly in a stack. A probe is utilized to move inwardly a predetermined distance to bend one or a plurality of disks downward out of its normal plane of rotation. The accessing arm which has a recording head at one end is then moved inwardly to read or record on the underside of a predetermined disk opposite the disks that have moved downward.
It should be observed that neither of the above prior art citations has the capability of reading or recording information on opposite sides of the disk without first going through another search cycle to locate the side opposite the side first magnetically written or read upon.
Other prior art such as U.S. Pat. No. 3,703,713 and an IEEE Transactions on Magnetics of September 1972 pertain to a selection scheme wherein a disk is accessed by bringing a picker to a desired radial position until it isolates the disk to be accessed from adjacent disks. The head is then moved radially inward to a desired track location on the disk. A recognized shortcoming of the selection scheme disclosed in the patent is that although there is separation between disks of approximately 60 mils, nevertheless if the disks are flopping away from the head, the disk will be missed. On the other hand, if the disks are flopping towards the head, several disks may be picked up.
The above publication although similar in operation to the patent nevertheless is distinguished therefrom by the fact that the disks of a group are in a touching relationship with one another as determined by dimensions set forth in FIG. 1 thereof. Furthermore, the disks are rotated at relatively high speed. These operational characteristics are not conductive to making a viable product in a commercial world. Thus, when the disks are in a touching relationship, wear particles that are developed during a read/write operation have no way of escaping. Wear particles are deleterious to the proper operation of the read/write cycle since they tend to ruin the disks as well as the head.
The above discussed shortcoming is also deemed to be characteristic of the above-mentioned patent since without high speeds the thin flexible disks would not become sufficiently rigid and would tend to droop and to flop. Accordingly, without a high speed rotation of the disks as shown in the patent, proper selection would not be feasible.